Stable System
An input is said to be bounded if it is bounded above and below for all values of t. For example, cos(t) is a bounded input since it is bounded above by 1 and below by -1, while exp(t) is not a bounded input since for increasing t, the function increases without bound. A system is therefore said to be bounded if a bounded output yields a bounded input. According to Professor Boutin, mathematically this means that there exists an $ \epsilon \! $ such that
$ |x(t)| < \epsilon \! $ for all values of t,
and then there exists an $ M\! $ for the ouput signal y(t) such that
